Impact absorbing members are used in automobiles, trains, ships, and other transport machines. These impact absorbing members can absorb the energy of impact by deforming under the impact load received at the time of impact and enable safety of the passengers along with this. As such impact absorbing members, for example, there are frame members or crush boxes of automobiles.
FIG. 1 is a perspective view schematically showing the arrangement of frame members and crush boxes in an automobile. As shown in this figure, at the four sides of an automobile, a front side member 2, rear side member 3, and side seals 7 are arranged. These members are all provided along the front-rear direction of the automobile. The front side member 2 is arranged at the front part of the four sides of the automobile, the rear side member 3 is arranged at the rear part of the four sides of the automobile, and the side seals 7 are arranged at the middle parts in the four sides of the automobile.
At the center part of the automobile in the front-rear direction, a floor is provided. On the floor, floor cross members (4, 4′) are arranged. The floor cross members (4, 4′) extend in the extension direction of the automobile.
Crush boxes (1a, 1b) are arranged at the front end of the frame formed by the above-mentioned frame members. More specifically, a first crush box 1a is provided at the front end of the front side member 2, while a second crush box 1b is provided at the rear end of the rear side member 3.
The frame members of these front side member 2, rear side member 3, side seal 7, and floor cross members (4, 4′) and the crush boxes (1a, 1b) sometimes receive loads in their axial directions at the time of impact. In this case, these members deform by buckling so as to be compressed in the axial direction in a bellows like manner and thereby absorb the impact load.
Such impact absorbing members can be fabricated by bending or overlay welding of metal sheet materials. Impact absorbing members fabricated from metal sheet are tubular. That is, they have closed cross-sectional shapes vertical to the axial direction. For this reason, the impact absorbing members are hollow at the insides.
Various proposals have been made in the past for an impact absorbing member absorbing an impact load by periodic buckling.
PLT 1 describes a crush box. This crush box is provided with not only the member forming the hollow cross-section, but also a center sheet extending in a horizontal manner so as to partition the hollow region to a top and bottom section near the center in the vertical direction of the hollow cross-section. By providing the center sheet, the crush box is made to not buckle and be crushed at the time of impact and to be kept from bending. In this example of the configuration, the members forming the hollow cross-section (first member and second member) and the third member forming the center sheet are all the same in sheet thickness of the metal sheet.
PLT 2 also describes a crush box. The crush box is arranged between a front side member and a bumper member at the front side of a vehicle. Further, the crush box is provided with a tubular shaped box main body and a reinforcement connecting a pair of facing wall parts of the box main body facing each other. If providing the reinforcement at the hollow part of the box main body, when setting the performance of the crush box to the target performance, it is considered possible to set it to the desired performance by the shape or size of the reinforcement. In this PLT 2, the sheet thickness of the metal sheet used for the box main body and the reinforcement are not studied at all.